Generally, cellulose acetate is a semi-synthetic polymer obtained from cellulose as a starting material, by esterification with acetic anhydride. Currently, commercially available cellulose acetate can be roughly divided into two groups according to its degree of acetylation. One is cellulose triacetate (hereinafter referred to as CTA) having a degree of acetylation not less than 59%, and the other is cellulose diacetate, which ranges widely, and that with a degree of acetylation of about 50 to 59% is referred to as cellulose diacetate (CDA). In other words, it is cellulose acetate soluble in acetone.
Cellulose acetate, especially CTA, which has excellent physical properties, particularly good processability and high optical properties, has been utilized for many years in the field of plastics, fibers and films (for example, photographic film, etc.). Further, cellulose acetate has attracted attention from the viewpoint of the global environment, because it possesses a biodegradability and the like.
A molded product of cellulose acetate, such as CTA, can be prepared generally by the fluidization of a solution of cellulose acetate dissolved in a solvent to a desired form, followed by removal of the solvent, for example, by evaporation (see, for example, JP-B 45-9074, JP-B 49-4554, JP-B 49-5614).
On the other hand, as the uses of cellulose acetate increase, speeding up of its processing is required and high speed molding, high speed spinning and high speed processing of the molded product have been tried. For example, in a process for producing a film, it is proposed to cast a solution of cellulose acetate at a high speed to be molded into a film. In order to improve the moldability corresponding to such speeding up, it is proposed to reduce the viscosity of a concentrated solution of cellulose acetate. In order to reduce the viscosity of such a concentrated solution, in general, the average degree of polymerization of the cellulose acetate has been reduced. However, when cellulose acetate with a low average degree of polymerization is used, the mechanical strength of the molded product will be impaired ("Effect of the degree of polymerization of CTA on the physico-mechanical properties of thread", Krim Volokna, 1985, No. 3, p. 46-47, etc.). Particularly, it is difficult to reduce the solution viscosity for cellulose acetate with a high degree of acetylation, especially CTA, while retaining a high degree of polymerization.
Further, a molded product of cellulose acetate is generally rigid and brittle, and these properties become more remarkable as the acetylation degree increases. The physical properties of certain polymeric material greatly depend on its crystallizability. That is, those having a high crystallizability are imparted with strength while flexibility, for example, elongation is impaired, resulting in a brittle product. Of course, CTA is not an exception, and has a high crystallizability due to its homogeneous structure. That is, for a cellulose acetate, the higher the degree of acetylation becomes, the higher the crystallizability will be. Moreover, crystals are generally formed because lower molecular weight materials act as nuclei. Accordingly, when CTA or CDA is used, the molded product is generally imparted with a flexibility by adding a plasticizer. For example, phthalate plasticizers, such as diethyl phthalate, may often be used for acetate plastics used as a grip of a screwdriver or the like. In addition, a cellulose acetate, particularly CTA, has a utility as a raw material for various films due to its excellent transparency. However, it has defects, for example, the film is rigid and brittle. To overcome these physical defects, a plasticizer is also used in this case. Adding ingredients such as a plasticizer is accompanied, not only with a decreased yield of the final product due to bleedout during molding, but also with economical disadvantages. Thus, the development of materials for producing a film having excellent physical properties while maintaining the characteristics of CTA by addition of a small amount of a plasticizer has been expected.
Accordingly, an object of the present invention is to provide a cellulose acetate having a low solution viscosity and excellent moldability, in spite of its high molecular weight and degree of polymerization.
Another object of the present invention is to provide a cellulose acetate having a high solubility in a solvent and high moldability, in spite of having a high average molecular weight and high average degree of substitution.
Still another object of the present invention is to provide a cellulose acetate useful in producing molded products having a high moisture resistance and dimensional accuracy by a molding process at a high processing speed using a solution of a cellulose acetate having a low solution viscosity.
Still another object of the present invention is to provide a process for producing a cellulose acetate having the excellent properties described above.